Combining Δ18o Isotope Data and In-Situ La-Icp-Ms Trace Element Analysis of Magnetite as a Proxy for Ore Genesis: Constraints on the Formation of Fe Deposits from Ossa-Morena Zone (Sw Iberian Peninsula)

Abstract

The Ossa-Morena Zone (OMZ, SW of the Iberian Peninsula) is a geotectonic domain that comprises a set of diversified ore deposits formed from Cambrian to Carboniferous-Permian ages. The Montemor-o-Novo – Ficalho Fe-Zn-(Pb) belt was a productive mining sector until the first half of the 20th century with potential for future mineral exploration, although the mechanisms responsible for ore deposition are, in some cases, poorly constrained. In this study the trace element composition of a large set of orebodies belonging to three iron deposits is examined, and the first δ18O analysis of magnetite from the Portuguese sector of OMZ is reported.

Outcrop and drill core magnetite samples were collected from the Montemor-o-Novo Iron Complex (MIC), and from the Alvito and Azenhas-Orada deposits. New magnetite LA-ICP-MS data from the carbonate-hosted MIC deposits, classified as SEDEX-VMS, and from the Alvito and Azenhas-Orada skarn deposits are complemented by previously published data. This approach contributes to the classification of these deposits and provides new discriminatory proxies for skarn deposits worldwide. Close to 1000 LA-ICP-MS and EPMA magnetite spot analyses is explored, complemented by EPMA analysis of representative mineral phases from the host rocks. Three deposits from the MIC (Monges, Vale da Arca, and Serrinha) were studied, in which magnetite reveals a wide range of textures and trace element compositions. Using the Al + Mn vs Ti + V diagram the magnetite from the MIC is hardly discriminated from the magnetite data from the skarn deposits. Hence other discriminant factors have been used. Magnetite from the MIC is characterized by low contents of Co, Zn, and HFSE (e.g. Ta, Nb), and shows a close relation with low-temperature hydrothermal magnetite as revealed by the multivariate diagrams of trace elements. Skarn magnetite display high concentrations of temperature-dependent elements such as Ti, V, Al, Ga, Sn, Cr, and HSFE. Additionally, high contents of Co and Zn are ubiquitous for first generation magnetite from the skarn deposits, which might indicate that the ore source was enriched in such elements.

Primary magnetite from the MIC deposits revealed δ18O signatures in the range 4.3 ‰ to 9.0 ‰ which, combined with magnetite trace element composition, suggests extensive overprinting of the primary signatures by late metamorphic events. Primary magnetite from the Azenhas-Orada deposits revealed δ18O signatures (4.0 ‰ – 5.6 ‰) suggesting a magmatic derived source for the mineralization, corroborating the hypothesis proposed in recent works. The δ18O signatures of magnetite from the Alvito skarn deposit indicate that extensive interactions between a magmatic fluid, with lighter 18O signatures, and the dolomite-calcite host rock, with heavier 18O signatures, have occurred. The results outline the potential of allying powerful multielement analysis of magnetite with stable isotope analysis and contribute to the refinement of geological models in the Ossa-Morena Zone that can be used in future mineral exploration.